Philippe Grossel

DC thermal microscopy: study of the thermal exchange between a probe and a sample

The Scanning Thermal Microscopic (SThM) probe, a thin Pt resistance wire, is used in the constant force mode of an Atomic Force Microscope (AFM). Thermal signal-distance curves for differing degrees of relative humidity and different surrounding gases demonstrate how heat is transferred from the heated probe to the sample. It is known that water affects atomic force microscopy and thermal measurements; we report here on the variation of the water interaction on the thermal coupling versus the probe temperature. Measurements were taken for several solid materials and show that the predominant heat transfer mechanisms taking part in thermal coupling are dependent on the thermal conductivity of the sample. The results have important implications for any quantitative interpretation of thermal images made in air.

D.C. scanning thermal microscopy : Characterisation and interpretation of the measurement

Abstract The used Scanning Thermal Microscopy (SThM) probe is a thin Pt resistance wire acting as a heat source and as a detector simultaneously. Its energetic balance is investigated by the study of the temperature profile along the probe. A theoretical approach of the measurement, based on this investigation, is then proposed. Simulations with this modelling are shown to predict how the heat, electrically produced in the probe, is dissipated in the probe-sample system. In particular, it is shown that the steady-state of conduction losses to the thermal element support varies versus the thermal conductivity of the sample and can lead to bad interpretations of the measurement.

AC thermal microscopy: a probe - sample thermal coupling model

A simple model of the thermal coupling between a probe tip and a sample is given, assuming an exchange through a thermal conductive medium. Comparisons between one-dimensional and three-dimensional calculations show the near-field character of the ac exchange. The coupling is analysed according to the different important parameters: probe - surface distances, sample thermal conductivity and diffusivity.

A.C. scanning thermal microscopy: Tip–sample interaction and buried defects modellings

Abstract Modellings describing the different thermal interactions between a sample surface and a probe in case of modulated heated probe are given and analysed. To obtain more reliable thermal data for a full calibration of thermal microscope or quantitative interpretations, these models are developed in three dimensions. The scattering of the thermal waves on buried small thermal resistances is theoretically studied. Lateral resolutions for the amplitude and phase of the probes a.c. thermal response are obtained.

3D thermal wave scattering on buried inhomogeneities in ac thermal microscopy

The scattering of the thermal waves on buried small thermal resistances is theoretically studied in the framework of conductive thermal modelling of the coupling between a SThM probe and a sample. Lateral resolutions for the amplitude and phase of the probes ac thermal response are obtained. The modelling, using an iterative method, goes beyond the usual first Born approximation results. Technically, the ac temperature field in the entire geometry is expressed in terms of two-dimensional plane wave spectra which permit direct use of fast Fourier transform algorithms. The various analytical results are easily obtained via a multilayer treatment of the probe and of the sample.

Non destructive testing in situ, of works of art by stimulated infra-red thermography

This paper presents various examples of assistance to the restoration of mural paintings by infra-red photothermal radiometry. First, we present the experimental device implemented for the study. Then, we show the possibility to detect separation or air voids by this technique in various works of art as the Saint Christopher of the Campanna collection of Louvre, in the painted ceilings of the abbey of Savin Saint sur Gartempe (classified with the world heritage of UNESCO) and finally in the Cocteau frescoes of the vault Saint Pierre of Villefranche sur mer.

Approach of the measurement of thermal diffusivity of mural paintings by front face photothermal radiometry

This paper deals with the approach of the possibilities of the front face photothermal thermography to measure the longitudinal thermal diffusivity of a mural painting. We present at first the principle of the method of measure based on the use coupled of transformed integrals and thermal quadruples. We show then the feasibility of the method by means of numerical simulation. We present then the experimental device implemented for the study. We show finally, by means of the experimental study of a sample of plaster, which the photothermal method allows in a particular case, a good estimation of the parameter longitudinal thermal diffusivity. Within the framework of the assistance to restoration of mural painting, our laboratory works since about ten year on the detection of detachment and air pocket situated in frescoes. The front face photothermal thermography, has already allowed us to detect, in situ, detachment situated in the Saint Christopher of the “Campana” collection of “Louvre”, in walls painted of the church Saint Florentin of Bonnet, in ceilings painted of abbey of “Saint Savin sur Gartempe” (classified in the UNESCO world heritage) and finally in the frescoes Cocteau of the vault “Saint Pierre” of “Villefranche sur Mer” [1-3 ]. These qualitative studies being positive, they push us now to study the possibilities of the photothermal method in characterization of the defects. Later, the objective is to determine both the area of the defect but also the depth in which it is situated. For it we intend to proceed by an adjustment theory / experiment with the use of methods of inverse techniques of the type levenberg marquardt [4]. To feed the model evoked previously, it is necessary to know the thermophysical properties of the studied materials. Two solutions were offered to us: either appeal to bibliographical values that is to develop a method of measure of diffusivity usable in situ. For reasons of appropriate life of materials (ageing, presence of humidity, manufacturing processes, environment), it is the second solution that we chose to implement. To lead to these measures of diffusivity, we had even there two solutions: either take a sample of work of art or implement a classic method of measure of this thermophysical parameter (method flash [5]), or develop a useful method in situ and non-destructive for the studied mural painting. For reasons of good preservation of the works of art, it is this last option we chose to implement. The thickness of the couple coats - wall composing the mural painting being often of several tens centimeters, a measure of transverse thermal diffusivity is often impossible. We thus chose to develop a method of measure of longitudinal thermal diffusivity of the work of art. It is this method we present here. We present at first the principle of the method of measure, based on the use coupled of integral transformations and the quadrupole method [6]. We show then the feasibility of the method by means of numerical simulation. We present then the experimental device developed for the study. We show finally, the method gives access to a good estimation of the longitudinal diffusivity of a sample of plaster. 2. Principle of in situ measure of longitudinal thermal diffusivity

Sample tip thermal coupling in modulated laser surface excitation

A model of the a.c. thermal coupling between a sample surface and a non-contact material probe, in the case of a modulated heated sample, is analyzed. The temperature field of the sample surface is obtained in using approximated self-consistent perturbation from the heat losses via the probe channel. Numerical estimation is given for uniform and local heat sources highlighting the roles of the modulation frequency and the probe dimensions.

Photothermal effects induced by laser excitation in a scanning tunneling microscope

Abstract Numerous near-field experimental methods use laser excitation inducing non-negligible thermal effects that are not taken into account. We particularly investigate the case of a scanning tunneling microscope. The analysis of the experimental results gives the thermal contribution of the probe in the measured signal when the probe is irradiated by the laser beam. The phase of the signal allows the distinction between the effects induced by the probe and the sample. We show that the experimental set-up can measure the periodic thermal expansion of the sample with the picometer resolution.

Temperature sensor for scanning thermal microscopy based on photoluminescence of microcrystal

A new sensor is developed for measuring local temperatures. This sensor is based on a thermal-resistive probe and on photoluminescence of crystal. The final purpose is to develop a device calibrated in temperature and capable of acquiring images of local temperature at sub-micrometric scale. Indeed, the sensor temperature can be obtained in two distinct ways: one from the thermal probe parameters and the other from the green photoluminescence generated in the anti- Stokes mode by the Er ions directly excited by a red laser. The thermal probe is in Wollaston wire whose thermal-resistive element is in platinum/rhodium. Its temperature is estimated from the probe electrical characteristics and a modelling. A microcrystal of Cd0.7Sr0.3F2: Er3+(4%)-Yb3+(6%) about 25μm in diameter is glued at the probe extremity. This luminescent material has the particularity to give an emission spectrum with intensities sensitive to small temperature variations. The crystal temperature is estimated from the intensity measurements at 522, 540 and 549 nm by taking advantage of particular optical properties due to the crystalline nature of Cd0.7Sr0.3F2: Er3+-Yb3+. The temperature of probe microcrystal is then assessed as a function of electric current in the thermal probe by applying the Boltzmanns equations. The first results will be presented and discussed.